Related Rate problem

Gravel is being dumped from a conveyor belt at a rate of 11 m^3/min The coarseness is such that it forms a pile in the shape of a cone whose base diameter and height are in the ratio of 5 to 2 . How fast is the height of the pile increasing when the pile is 6 metres high?

When I finally plug in 6 for h, I get −.0587184655608
which appears to be wrong!